Focused Ion Beam (FIB) microscope systems have been produced commercially since the mid 1980's, and are now an integral part of rapidly bringing semiconductor devices to market. FIB systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the scanning electron microscope, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. These ions are, in general, positively charged.
These ion beams, when directed onto a semiconductor sample, will eject secondary electrons, secondary ions (i+ or i−), and neutral molecules and atoms from the exposed surface of the sample. By moving the beam across the sample and controlling various beam parameters such as beam current, spot size, pixel spacing, and dwell time, the FIB can be operated as an “atomic scale milling machine,” for selectively removing, or sputtering, materials wherever the beam is placed. The dose, or amount of ions striking the sample surface, is generally a function of the beam current, duration of scan, and the area scanned. The ejected particles can be sensed by detectors, and then by correlating this sensed data with the known beam position as the incident beam interacts with the sample, an image can be produced and displayed for the operator.
FIG. 1 is a schematic of a typical FIB system. FIB system 10 includes an evacuated envelope 11 having an upper neck portion 12 within which are located a liquid metal ion source 14 and a focusing column 16 including extractor electrodes and an electrostatic optical system. Ion beam 18 passes from source 14 through column 16 and between electrostatic deflection means schematically indicated at 20 toward sample 22, which comprises, for example, a semiconductor device positioned on movable X-Y stage 24 within lower chamber 26. An ion pump 28 is employed for evacuating neck portion 12. The chamber 26 is evacuated with turbomolecular and mechanical pumping system 30 under the control of vacuum controller 32. The vacuum system provides within chamber 26 a vacuum of approximately 1×10E-7 Torr. If an etch assisting gas, an etch retarding gas, a deposition precursor gas, or some other reactive or non reactive gas is used, the chamber background pressure may rise, typically to about 1×10E-5 Torr.
High voltage power supply 34 is connected to liquid metal ion source 14 and to appropriate electrodes in focusing column 16 and directing the ion beam. Deflection controller and amplifier 36, operated in accordance with a prescribed pattern provided by pattern generator 38, is coupled to deflection plates 20. A charged particle multiplier detector 40 detects secondary ion or electron emission for imaging, is connected to video circuit and amplifier 42, the latter supplying drive for video monitor 44 also receiving deflection signals from controller 36. A door 48 is provided for inserting sample 22 onto stage 24, which may be heated or cooled. Focused ion beam systems are commercially available from various companies, but the system shown in FIG. 1 represents one possible FIB system configuration.
During any beam raster operation executed by FIB system 10, which includes imaging, milling, gas assisted etching or deposition, the FIB beam deflection software and hardware deflects the beam in a preset pattern across the surface, generally referred to as rastering. At each preset location, the beam is left to dwell for a given period of time before moving to the next point in the raster. At its simplest, a raster pass consists of deflecting the beam at fixed increments along one axis from a start point to an end point, dwelling for a fixed dwell time at each point. At the end of a line, the beam waits a fixed retrace time before moving an increment in a second axis. The beam may return to the start point in the first axis and begin again, or may begin “counting down” the first axis from the point it had just reached (depending on whether the raster type is raster (the former) or serpentine (the latter). This process continues until all increments in both axes have occurred, and the beam has dwelled at all points in the scan.
It is well understood by those of skill in the art that FIB systems are used to perform microsurgery operations for executing design verification or to troubleshoot failed designs. This can involve physically “cutting” metal lines or selectively depositing metallic lines for shorting conductors together. Hence, FIB system technologies can enable prototyping and design verification in a matter of days or hours rather than weeks or months as re-fabrication would require. This FIB “rapid prototyping” is frequently referred to as “FIB device modification”, “circuit editing” or “microsurgery.” Due to its speed and usefulness, FIB microsurgery has become crucial to achieving the rapid time-to-market targets required in the competitive semiconductor industry.
The success of any FIB microsurgery operation depends on the precise control of the milling process, and the position on the semiconductor circuit at which milling is to occur. An unintentionally cut metal line, or deposition of material in the wrong area can render the semiconductor circuit defective. Current integrated circuits have multiple alternating layers of conducting material and insulating dielectrics, with many layers containing patterned areas. Hence it can be challenging for a FIB operator to identify the precise location where an operation is to occur, especially if the structure of interest resides underneath a layer of conducting and/or insulating material. Therefore, proper navigation through visual imaging of the semiconductor device is necessary. However, a FIB is not an ideal means for generating images for navigation due to its inherent destructive process and surface sensitive nature.
Field emission scanning electron microscopes (FESEM) on the other hand, utilize and electron beam for imaging materials. FIG. 2 is an illustration of a typical electron beam system.
The electron beam system of FIG. 2 includes an electron beam column 60, a specimen vacuum chamber 62, a reactant material delivery system 64, and a user control station 66. An electron beam 68 is emitted from a cathode 70 by applying voltage between cathode 70 and an anode 72. Electron beam 68 is focused to a fine spot by means of a condensing lens 74 controlled by a condensing lens control circuit in user control station 66 and an objective lens 76 controlled by an objective lens control circuit in user control station 66. Electron beam 68 is scanned two-dimensionally on the specimen by means of a deflection coil 78 controlled by a deflection control circuit in user control station 66. Electron beam 68 is focused onto a work piece 80. Work piece 80 is located on a movable stage 82 within the specimen vacuum chamber 62. The specimen vacuum chamber 62 includes a secondary electron detector 84 for detecting secondary particles suitable for generating an image of the work piece.
The operation of secondary electron detector 84 is controlled by a control unit within user control station 66. Secondary electron detector 84 is also connected to an amplifier (not shown). The amplified signals are converted into digital signals and subjected to signal processing by a signal processor for providing a resulting digital signal which is used by a CPU, to display an image of the work piece 80 on a monitor. The reactant material delivery system 64 includes a reservoir connected to a delivery conduit 86 for delivering reactant materials to the surface of work piece 80. It is noted that most commercially available FESEM machines do not have gas delivery systems, however, dual beam systems (incorporating FIB and SEM columns in the same chamber) will inherently have a gas delivery system for supporting FIB circuit etch and deposition operations.
Electrons essentially cannot sputter material on their own because the momentum of an electron in a typical electron beam is not sufficient to remove molecules from a surface by momentum transfer. The amount of momentum that is transferred during a collision between an impinging particle and a substrate particle depends not only upon the momentum of the impinging particle, but also upon the relative masses of the two particles. Maximum momentum is transferred when the two particles have the same mass. When there is a mismatch between the mass of the impinging particle and that of the substrate particle, less of the momentum of the impinging particle is transferred to the substrate particle. A gallium ion used in focused ion beam milling has a mass of over 130,000 times that of an electron. In a typical focused ion beam system, the gallium ions are accelerated through a voltage of 25-50 kV, whereas the electrons in a scanning electron microscope are typically accelerated through a voltage of 1 kV to 30 kV, but most often at 5 kV. The momentum transfer of a typical 30 kV gallium ion impinging on a copper substrate in a FIB system is therefore greater than 1000 times that of a 5 kV electron in an electron microscope.
An electron beam will not etch in the absence of a chemical etchant, whereas an ion beam will always sputter material, even though sputtering may be enhanced or attenuated in the presence of a gas. Therefore, an electron beam cannot be used to etch a particular material unless a specific chemical is used that will etch the material in the presence of the electron beam. Furthermore, the specific chemical may not significantly etch the material in the absence of electron beam. The use of electron beams to etch a variety of materials in the presence of specific chemicals is known the art.
When a primary electron beam is directed onto a sample, the electrons impinging on the sample react with the sample and cause electrons to emanate from the sample. According to the characteristics of the sample at the position at which the primary electron beam impinges thereupon, more or less electrons will, at constant primary electron beam intensity, emanate from the sample. From an examination of the intensity of the electrons emanating from the sample in dependence of the location, at which the primary electron beam impinges on the sample, images may be obtained.
The electrons emanating from the sample are generated by the electrons of the primary electron beam through different physical effects. These effects can include the generation of back scattering electrons, which according to a common definition have an energy of more than 50 eV and are abbreviated BSE; the generation of electrons which have an energy of less than 50 eV and are termed secondary electrons in the narrower sense. These are discriminated into secondary electrons abbreviated SE1, which are generated near the surface of the sample by an impinging primary electron. Secondary electrons abbreviated SE2 can be generated by back scattering electrons emanating from the sample near the sample's surface; the generation of electrons of the primary electron beam, which do not quite reach the surface of the sample but are reflected just before the sample's surface due to a charging of the sample and are commonly referred to as mirror electrons; and the generation of transmission electrons, which are primary electrons traversing the sample and scattered primary electrons and secondary electrons emanating from the sample in a direction of the primary electron beam. Auger electrons can also be emitted from the sample.
Therefore, in addition to the use of FESEM's for non-destructive imaging and navigation of a semiconductor device, they can be used in the presence of an appropriate gas for selective etching of materials. By example, U.S. Patent Application Publication No. 2005/0072753A1 filed on Jul. 28, 2003 by Koops et al., describes a method for etching photo-masks and stencil masks that are used for semiconductor device fabrication patterning. Koops et al. uses an electron beam having a landing energy between 100 eV and 200 keV, with a beam resolution of 2 nm under typical conditions.
However, the relatively high landing energies of the electron beam may not be suitable for FESEM gas assisted etching of semiconductor circuits. Ideally, gas breakdown is more efficient at low landing energies of the electron beam. Those skilled in the field of low energy electron ionization sources used in gas chromatography and mass spectrometry, understand that broad beam electron impact ionization sources are optimized for ˜70 eV energy, however lower energies are often useful for providing more selective dissociation of the gas phase products. This is described in Gas Chromatography, 1960 R. P. W. Scott (Editor), Butterworths, London, 1960, xvii+466 pp., 955, an excerpted which is available on the Internet at http://www.chromatography-online.org/GC-Tandem/GC-MS/Ion-Generation/Electron-Impact-lonization/rs65.html. Scott states that the electron energy that will provide optimum ionization varies between different compounds, but an average value appears to fall within the range of 50 and 100 eV.
Unfortunately, there are very few FESEMs which can provide low landing energy electron beams. The Hitachi High Technologies model 4800 and the Carl Zeiss SMT Ultra 55 have been in production for a few years, and are designed to provide landing energies no lower than 100 V. This is still very far from the ˜20 V expected energy of the secondary electrons emitted from focused ion beam interaction processes with the target substrate, and is still above the 20 eV to 70 eV ideal range for gas decomposition, as previously described.
It is, therefore, desirable to provide a system for generating electron beams with low landing energies, and a method for using these generated low landing energies for circuit edit operations.